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Providing Insight
Into Climate Change
Climate Change Science Essay page 3

Sun Activity Correlates with Temperature

Numerous papers published in major peer-reviewed scientific journals shows the Sun is the primary driver of climate change.  There is a very strong correlation between the Sun activity and temperature.

Early in the nineteenth century, William Herschel (1738-1822), discoverer of Uranus, found that five periods of low number of sunspots corresponded to high wheat prices when the temperatures were cold. (Cold climate reduces the supply of wheat causing its price to rise.) See here.

E. Friis-Christensen and K.Lassen have shown that the length of the mean 11 year Sunspot cycle correlates to the northern hemisphere temperature during the past 130 years.  The length of the Sunspot cycle is known to vary with solar activity, whereas high solar activity implies short sunspot cycle length. See here for further information.

See here for an updated plot based on Friis-Christensen and Lassen's methodology.

Here is a correlation of the sunspot cycle length, global temperature and CO2 concentrations.
                              Sunspot Cycle Length Temperature and CO2

Sunspot cycle length

The red squares on the graph represent the sunspot cycle lengths. One point is the cycle length from the time of the maximum number of sunspots to the time of the maximum number of sunspots of the next cycle, and the following point is the cycle length from the time of the minimum number of sunspots to the time of the minimum number of sunspots of the next cycle. The sunspot cycles are back filtered using weighting 1,2,3,4 applied to each cycle point, both min to min and max to max. This assumes that the current cycle has the most effect on temperature (weight 4), and previous half cycles affect current temperatures in declining amounts, but future cycles have no effect on the current temperature. The temperature curve in blue used the HadCRUT3 land and sea data to 1978, the MSU satellite data from 1984 to 2006, and the average of the datasets for 1979 to 1983. This eliminates much of the urban heat island effects. The temperatures are unfiltered annual. The CO2 concentrations (ppmv) from 1958 to 2007 are derived from air samples collected at the Mauna Loa Observatory, Hawaii. CO2 concentrations prior to 1958 are uncertain.

Note that there is a correspondence between sunspot cycle length and temperature. Both the temperature and the cycle length curves begin to rise at 1910, and temperatures fall after 1945 to 1975 when the cycle length curve falls, and both curves rise again after 1975.  Temperatures have been increasing since 1980 faster than can be explained by the sunspot cycle length, indicating a possible human CO2 contribution. The recent increase of the cycle lengths explains why there has been no warming since 2002. Temperature changes are expected to follow Sun activity changes due to a time lag resulting from the large heat capacity of the oceans.

N. Scafetta of Duke University, Durham, NC and B.J. West of the US Army Research Office, NC studied the solar impact on 400 years of the Northern Hemisphere temperatures since 1600. They find good correspondence between temperature and solar irradiance proxy reconstructions up until 1920 as shown on the graph below.

                    Northern Hemisphere Temperature vs Solar Irradiance 400 years

Moberg NH temperature and TSI


The temperature curve is derived from proxy records to 1850 by Moberg et al. [2005], and from instrumental surface temperature data from 1850 to about 1980. The surface temperature record includes the urban heat island (UHI) and land use changes effects. The Northern Hemisphere MSU lower troposphere record is shown from 1979 in blue, which eliminates most of the UHI effects. Two different solar irradiance proxy reconstructions are shown: Lean, 2000; Wang et al., 2005. Both curves merge the ACRIM satellite data since 1980 with the proxy data. By assuming ACRIM, the solar activity has an increasing trend during the second half of the 20th century.  This graph is modified from the version created by Scafetta and West, which uses the contaminated instrument record after 1979 instead of the satellite data.  See the original version here.

Note the low solar activity periods occurring during the Maunder Minimum (1645 to 1715, the Little Ice Age) and during the Dalton Minimum (1795 to 1825).

Note the excellent correlation from 1600 to 1900 when humans were unlikely to effect climate. During the 20th century one continues to observe a significant correlation between the solar and temperature patterns:  both records show an increase from 1900 to 1950, a decrease from 1950 to 1970, and again an increase from 1970 to 2000.

A divergence of the curves from the Scafetta and West original graph indicates that the Sun is responsible for 56% using Lean 2000, and 69% using Wang 2005, of the northern hemisphere warming from 1900 to 2005. The authors estimate the error at 20%.

There are two solar composites available from satellite data. The ACRIM is obtained directly from the satellite data, while the PMOD assumes that Nimbus7/ERB satellite data covering the ACRIM gap (19891992) are still significantly corrupted and require additional severe adjustments. The ACRIM data shows higher solar irradiance during solar cycle 22 - 23 than the PMOD data.  Using the PMOD data and the original graph, the Sun likely has contributed 50% of the surface warming from 1900 to 2005.

The authors did a similar analysis using the Mann and Jones 2003 temperature reconstruction. This temperature history shows little variation before 1900 and shows a hockey stick shape. This reconstruction has been severely criticized for several reasons. See The IPCC Hockey Stick section of this essay.  The authors found that the Mann and Jones 2003 reconstruction (when compared to the Lean 2000 data) results in an unphysical zero response time to solar forcing. The ocean's large heat capacity should result in a time lag of surface temperatures with respect to long time solar changes of several years, so this reconstruction cannot be correct.

The authors' analysis shows the Sun has contributed 50 to 69% of the surface warming depending on the reconstructions utilized. The remainder may be due to CO2, UHI and land use changes. The authors compare the Sun's irradiance to the Northern Hemisphere land surface temperatures, which are contaminated with the urban heat island effect. The global MSU satellite temperatures, which are not contaminated by the UHI effect, have increased by half as much as the North Hemisphere temperatures since 1980. If the Scafetta and West analysis used the uncontaminated satellite data since 1980, the results would show that the Sun has contributed at least 75% of the global warming of the last century. See more about the UHI effect later in this essay here. See here for the November 2007 article.

Climate alarmists claimed that solar activity couldn’t possibly have anything to do with the warming of the late 20th century because sunspot numbers peaked about 1960 then decline while global temperatures rose over the 2nd half of the 20th century. The solar activity curve, which was updated in 2015, shows total solar irradiance peaked in 1990 with solar cycle 22. Solar activity isn’t just sunspot numbers. Lüning and Vahrenholt write “The sun not only reached its maximum at the end of the 20th century, but was apparently stronger than at any time over the past 10,000 years." The graph below shows sunspot numbers and total solar irradiance (TSI). See here.

TSI and Sunspots

A group of NASA and university scientists have found convincing evidence of a link between the Sun activity and climate by comparing the records of the historical water level of the Nile River to the number of auroras observed in northern Europe and the Far East between 622 and 1470 AD. Auroras are bright glows in the night sky following solar flares, and are an excellent means of tracking solar activity.  See this link for further information.
A study by WJR Alexander et al, published June 2007 compared hydrometeorological data to solar variability. The study looked at rainfall, river flow and flood data. The authors conclude that there is "an unequivocal synchronous linkage between these processes in South Africa and elsewhere, and solar activity." The study included an analysis of the level of Lake Victoria, which has been carefully monitored since 1896. In the early 1960s a dramatic rainfall increase significantly raised the lake level, and the level since then has been falling at about 29 mm per year. The decline has been removed from the data plotted below.  The plot shows two periods of strong correlation between lake level and sunspot number, corresponding to periods of high levels of volcanic dust.

                                 Lake Victoria Water Level and Sunspot Number

Lake Victoria Water Level and Sunspot Number


See the paper "Linkages between solar activity, climate predictability and water resource development" here.

Longer term, here is a correlation of a solar proxy to a temperature proxy for a period of 3000 years. Values of carbon-14 (produced by cosmic rays hence a proxy for solar activity) correlate extremely well with oxygen-18 (temperature proxy).  The lower graph shows a particularly well-resolved time interval from 8,350 to 7,900 years BP.

Solar activity and climate 3500 years

The above graph summarizes data obtained from a stalagmite from a cave in Oman, as reported in the paper, Neff, U., et al. 2001.

A team of researchers led by scientists from the Max Planck Institute for Solar System Research analysed radioactive isotopes in trees and has found that the Sun has been more active in the last half of the 20th century than in any time in the last 8000 years. This study showed that the current episode of high solar activity since about the year 1940 is unique within the last 8000 years. See a press release here. A graph from the study is below. The bottom chart is a detail of the shaded period of the top chart from 9300 to 8600 years before the present.

Solar Activity 11,500 Years

A study published by the Danish Meteorological Institutecompares the Koch ice index which describes the amount of ice sighted from Iceland, in the period 1150 to 1983 AD, to the solar cycle length, which is a measure of solar activity. The study finds "A close correlation (R=0.67) of high significance (0.5 % probability of a chance occurrence) is found between the two patterns, suggesting a link from solar activity to the Arctic Ocean climate."

Koch ice index and Solar Cycle Length

See here.

Tim Patterson, an adviser to the FOS,  has studied high-resolution Holocene climate records from fjords and coastal lakes in British Columbia and demonstrates a link between temperature and solar cycles.

Patterson Spectral analysis




The spectral analysis shown here is from sediment cores obtained from Effingham Inlet, Vancouver Island, British Columbia. The annually deposited laminations of the core are linked to the changing climate conditions. The analysis shows a strong correlation to the 11-year sunspot cycle.

See here for a powerpoint slide show by Tim Patterson.

N. Shaviv and J. Veiser using seashell thermometers shows a strong correlation between temperature and the cosmic ray flux over the last 520 million years.
       Cosmic Ray Flux and Tropical Temperature Variation Over the Phanerozoic 520 million years

Cosmic Ray Flux and Tropical Temperature Variation


The upper curves describe the cosmic ray flux (CRF) using iron meteorite exposure age data. The blue line depicts the nominal CRF, while the yellow shading delineates the allowed error range. The two dashed curves are additional CRF reconstructions that fit within the acceptable range. The red curve describes the nominal CRF reconstruction after its period was fine-tuned to best fit the low-latitude temperature anomaly. The bottom black curve depicts the smoothed temperature change derived from calcitic shells over the Phanerozoic. The red line is the predicted temperature model for the red curve above. The green line is the residual.  The top blue bars indicate ice ages.

A paper by Nicola Scafetta (2011) compares the historical records of mid-latitude auroras from 1700 to the surface temperature records. It shows that auroras record share the same ocsillation frequencies evident in the temperature record and in several planetary and solar records.The author argues that the aurora records reveal a physical link between climate change and astronomical oscillations. The abstract states "In particular, a quasi-60-year large cycle is quite evident since 1650 in all climate and astronomical records herein studied ... The existence of a natural 60-year cyclical modulation of the global surface temperature induced by astronomical mechanisms, by alone, would imply that at least 60 to 70% of the warming observed since 1970 has been naturally induced. Moreover, the climate may stay approximately stable during the next decades because the 60-year cycle has entered in its cooling phase."See here .

A paper published in "Nature Geoscience" in March 2014 titled "Solar Forcing of North Atlantic Surface Temperature and Salinity Over the Past Millennium" found that solar activity correlates well with North Atlantic temperatures. The graph below shows the temperature reconstruction and total solar irradiance, with a 12.4 year lag applied.

Northern Atlantic sea temperature and total solar irradiance

The abstract states, "There were several centennial-scale fluctuations in the climate and oceanography of the North Atlantic region over the past 1,000 years, including a period of relative cooling from about AD 1450 to 1850 known as the Little Ice Age. These variations may be linked to changes in solar irradiance, amplified through feedbacks including the Atlantic meridional overturning circulation. ...
low solar irradiance promotes the development of frequent and persistent atmospheric blocking events, in which a quasi-stationary high-pressure system in the eastern North Atlantic modifies the flow of the westerly winds. We conclude that this process could have contributed to the consecutive cold winters documented in Europe during the Little Ice Age." See here.

A paper by Soon et al 2015 finds a strong correlation between Northern Hemisphere (NH) exo-tropic temperatures and total solar irradiance (TSI). The NH temperatures were determined by using mostly rural stations to remove the effects of urban development that contaminates government datasets. The authors used the solar variability dataset by Scafetta & Willson, 2014 to represent TSI. The graph below shows a correlation of R2= 0.48, implying that solar variability has been the dominant influence on Northern Hemisphere temperature trends since at least 1881.

Total solar irradiance and N. Hemisphere temperature composite

The Sunspot Cycle 24 has a smoothed sunspot number maximum of about 100 in late 2013. NASA's Cycle 24 Sunspot number prediction graph, showing three cycles is shown below.

Cycles 22, 23, 24 sunspot numbers

NASA's Solar Cycle Prediction page [no longer exists] says "The current predicted and observed size makes this [cycle 24] the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906."  The NOAA solar cycle image is here.

A new model of the sun has produce unprecedentedly accurate predictions of the sun's variable solar cycles. The model uses two solar dynamos, one near the solar surface and one in the deeper in the convection zone. The model was described in a paper by Shepherd et al 2014 here and described here.  The model predicts that solar activity will fall from cycle 24 activity by 60 per cent during the 2030s to conditions last seen during the 'mini ice age' that began in 1645.

Sun and Cosmic Rays

During the 20th century the Sun has continued to warm and may have contributed directly to a third of the warming over the last hundred years. The change in solar output is too small to directly account for most of the observed warming. However, the Sun-Cosmic Ray connection provides an amplification mechanism by which a small change in solar irradiance will have a large effect on climate.

A paper by H. Svensmark and E. Friis-Christensen of the Center for Sun-Climate Research of the Danish National Space Center in Copenhagen has shown that cosmic rays highly correlate to low cloud formation. Changes in the intensity of galactic cosmic rays alter the Earths cloudiness. 

An experiment in 2005 shows the effect of cosmic rays in a reaction chamber containing air and trace chemicals found over the oceans. Electrons released in the air by cosmic rays act as a catalyst in making aerosols. They significantly accelerate the formation of stable, ultra-small clusters of sulphuric acid and water molecules, which are the building block for the cloud condensation nuclei.

Danish scientists reported in May 2011 that they have succeeded for the first time in directly observing that the electrically charged particles coming from space and hitting the atmosphere at high speed contribute to creating the aerosols that are the prerequisites for cloud formation. In a climate chamber at Aarhus University, scientists have created conditions similar to the atmosphere at the height where low clouds are formed. This artificial atmosphere was irradiated with fast electrons from ASTRID Denmarks largest particle accelerator. The experiments show that increased radiation from cosmic rays leads to more aerosols. In the atmosphere, these aerosols grow into actual cloud nuclei in the course of hours or days. Water vapour concentrates on the nuclei forming small cloud droplets. See the news release here.

A team of 63 scientists published results in August 2011 of a much more sophisticated experiment which investigated the effects of cosmic rays on cloud formation. The CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN (European Organization for Nuclear Research) in Geneva show big effects of pions from an accelerator, which simulate the cosmic rays and ionize the air in the experimental chamber. The CLOUD experiment is the most rigorous test of the Cosmic Ray hypothesis yet devised. The experiments show that cosmic rays strongly enhance the formation rate of aerosols by up to ten fold, and confirms the earlier results from the Danish experiment. The aerosols may grow into cloud condensation nuclei on which cloud droplets form. See the CERN press release here.

The graph below shows the aerosol particle concentration growth in the CLOUD chamber. In an early-morning experimental run at CERN, starting at 03:45, ultraviolet light began making sulphuric acid molecules in the chamber, while a strong electric field cleansed the air of ions. As soon as the electric field was switched off at 04:33, natural cosmic rays raining down through the roof helped to build clusters at a higher rate. When CLOUD simulated stronger cosmic rays with a beam of charged pion particles starting at 4:58 the rate of cluster production became faster still. The various colours are for clusters of different diameters (in nanometres) as recorded by various instruments. The largest (black) took longer to grow than the smallest (blue). See here. The CLOUD results also show that trace vapours assumed until now to account for aerosol formation in the lower atmosphere can explain only a tiny fraction of the observed atmospheric aerosol production.

CLOUD chamber aerosols

Coronal mass ejections from the sun cause a large decrease in the cosmic ray count, which are called Forbush decrease. These dramatic, short term cosmic ray decreases can be used to confirm the cosmic ray effects on clouds. The magnetic plasma clouds from solar coronal mass ejections provide a temporary shield against galactic cosmic rays.

A study by Svensmark et al in 2009 shows that the decrease in cosmic rays have a large effect on the amount of aerosols, cloud cover and the liquid water content of clouds. The authors conclude "From solar activity to cosmic ray ionization to aerosols and liquid-water clouds, a causal chain appears to operate on a global scale." 

The figure below shows the evolution of fine aerosols particles in the lower atmosphere (AERONET), cloud water content (SSM/I), liquid water cloud fraction (MODIS), and low IR-detected clouds (ISCCP), averaged for the 5 strongest Forbush decreases in the period 1987-2007. The red dashed line shows the average cosmic ray count percent change. The lowest aerosol count occurs 5 days after the Forbush minimum, and the cloud water content minimum occurs 4 days later.  The response in cloud water content for the larger events is about 7%.

The broken horizontal lines denote the mean for the first 15 days before the Forbush minimum of each of the four data sets.

Forbush decreases in the period 1987-2007


See here.

Data from the International Satellite Cloud Climatology Project and the Huancayo cosmic ray station shows a remarkable correlation between low clouds (below 3 km) and cosmic rays. There are more than enough cosmic rays at high altitudes, so changes in the cosmic rays do not effect high clouds. But fewer cosmic rays penetrate to the lower clouds, so they are sensitive to changes in cosmic rays.

                   Cosmic Rays and Low Clouds

Svensmark Low Cloud Comic Rays


The blue line shows variations in global cloud cover collated by the International Satellite Cloud Climatology Project. The red line is the record of monthly variations in cosmic-ray counts at the Huancayo station.

Low-level clouds cover more than a quarter of the Earth's surface and exert a strong cooling effect on the surface. A 2% change in low clouds during a solar cycle will change the heat input to the Earth's surface by 1.2 watts per square metre (W/m2). This compares to the total warming of 1.4 W/m2 the IPCC cites in the 20th century. (The IPCC does not recognize the effect of the Sun and Cosmic rays, and attributes the warming to CO2.)

Cosmic ray flux can be determined from radioactive isotopes such as beryllium-10, or the Suns open coronal magnetic field. The two independent cosmic ray proxies confirm that there has been a dramatic reduction in the cosmic ray flux during the 20th century as the Sun has gained intensity and the Sun's coronal magnetic field has doubled in strength.

                      Cosmic Ray Flux Since 1700

Cosmic Ray Flux Since 1700


Changes in the flux of galactic cosmic rays since 1700 are here derived from two independent proxies, 10Be (light blue) and open solar coronal flux (dark blue) (Solanki and Fligge 1999). Low cloud amount (orange) is scaled and normalized to observational cosmic-ray data from Climax (red) for the period 1953 to 2005 (3 GeV cut-off). Both scales are inverted to correspond with rising temperatures. Note that high cosmic ray flux around 1700 is at the end of the Little Ice Age. Also note the increase in cosmic ray flux after 1780 at the time of the Dicken's Winters.

The graph below shows a correlation between the cosmic ray counts and the global troposphere temperature radiosonde data. The cosmic ray scale is inverted to correspond to increasing temperatures. High solar activity corresponds to low cosmic ray counts, reduced low cloud cover, and higher temperatures. The upper panel shows the troposphere temperatures in blue and the cosmic ray count in red. The lower panel shows the match achieved by removing El Nino, the North Atlantic Oscillation, volcanic aerosols and a linear trend of 0.14 Celsius/decade.

troposphere temperature and cosmic rays

The negative correlation between cosmic ray counts and troposphere temperatures is very strong, indicating that the Sun is the primary climate driver. H. Svensmark and E. Friis-Christensen published the above graph in a paper October 2007 in response to a paper by M. Lockwood and C. Frohlich, in which they argue that the historical link between the Sun and climate came to an end about 20 years ago. However, the Lockwood paper had several deficiencies, including the problem that they used surface temperature data that is contaminated by the urban heat island effect (see below). They also fail to account for the large time lag between long-term solar intensity changes to the climate temperature response.

See the Svensmark rebuttal of the Lockwood paper here, and a critique by myself here.

Over the 20th century the Sun has increased activity and irradiance intensity, directly providing some warming. The graph below from here shows the rising solar flux during most of the twentieth century.

                                  Open Solar Flux

Open Solar Flux


Dr. U.R. Rao of Bangalore, India, shows that galactic cosmic rays, using 10Be measurements in deep polar ice as the proxy, has decreased by 9% during the last 150 years. The decrease in cosmic rays cause a 2.0% decrease in low cloud cover resulting in a radiative forcing of 1.1 W/m2, which is about 60% of that due to the CO2 increase during the same period. See here.

Neutrons, HMF, Sunspot

In the top panel showing cosmic ray intensity, the continuous line represents estimated Climax neutron monitor counting rate (1956-2000), open circles denote ionization chamber measurements during (1933-1956) and filled circles represent cosmic ray intensity derived from 10Be (1801-1932). 10Be is a long-lived radioactive beryllium isotope produced by cosmic rays. The middle panel shows the near-Earth helio-magnetic field and the lower panel shows the sunspot number.

A reconstruction of the near Earth heliospheric magnetic field strength from 1900 through 2009 from here by Svalgaard and Cliver (2010) is shown below.

heliospheric magnetic field strength

The red curve are satellite direct measurements of the near-Earth heliospheric magnetic field (HMF) strength resulting from the solar wind. The blue curve is the Inter-Diurnal Variability (IDV) index calculated from the geomagnetic field observations one hour after midnight. The IDV is highly correlated with the near-Earth HMF. The green curve are estimates of HMF by Lockwood et al 2009.

When the Sun is active it has a higher number of sun spots and emits more solar wind - a continuous stream of very high-speed charged particles. The increased solar wind and magnetic field repels cosmic rays that otherwise would hit the Earth's atmosphere, resulting in less aerosols in the lower atmosphere thereby reducing low cloud formation. The low clouds have a high reflectivity and have a strong cooling effect by reflecting sunlight back into space.

In summary, the process is:
More active Sun  -->  more Sunspots --> more solar wind --> less cosmic ray --> less aerosols --> less low clouds --> more sun light to the surface --> global warming.

The theory of CO2 warming implies that the arctic and Antarctica should be warming about the same, and the polar regions should be warming more that the rest of the Earth. However, Antarctica has not warmed since 1975, which is a big problem for the CO2 theory.  The ice covering Antarctica has even higher reflectivity than low clouds, so fewer low clouds cools Antarctica, while fewer low clouds warms the rest of the planet. (Greenland's ice sheet is much smaller and is not so reflective.) This Antarctica temperature trend is strong evidence that the Sun, not CO2, is the primary climate driver.

      Antarctica and North America Temperature Trends

Antarctica and North America Temperature Trends



The top curve is the North American surface temperature and the bottom curve is the Antarctica (64 S - 90 S) surface temperature over the past 100 years. The Antarctic data have been averaged over 12 years to minimize the temperature fluctuations. The blue and red lines are fourth-order polynomial fits to the data. The curves are offset by 1 K for clarity, otherwise they would cross and re-cross three times.

The cosmic ray flux is not only influenced by the solar wind, it also varies with the position of the solar system in the galactic arms. The solar system passes through the arms of the Milky Way galaxy roughly every 140 million years. When the solar system is in the galactic arms the intensity of cosmic rays increases, as we are closer to more supernovas that give off powerful bursts of cosmic rays. The variations of the cosmic ray flux due to the solar system passing through four arms of the Milky Way galaxy during the last 550 million years is ten times greater than that caused by the Sun. The correlation between cosmic rays and temperatures over 520 million years by N. Shaviv and J. Veiser was shown previously. Below is a similar graph based on their work, but with the times of the galactic arm crossings shown.

    Cosmic Ray Flux and Temperature Changes with Galactic Arm Crossings

Cosmic Ray Flux and Temperature Changes with Galactic Arm Crossings


Four switches from warm hothouse to cold icehouse conditions during the Phanerozoic are shown in variations of several degrees K in tropical sea-surface temperatures (red curve). They correspond with four encounters with spiral arms of the Milky Way and the resulting increases in the cosmic-ray flux (blue curve, scale inverted). (After Shaviv and Veizer 2003)

Temperature changes over this time range can not be explain by the CO2 theory.

                   CO2 Concentrations 500 Million Years

CO2 concentration 500 MM yr




The graph shows CO2 concentration over the last 500 million years. The CO2 does not correlate with temperature. Note when CO2 concentrations were more than 10 times present levels about 175 million years ago and 440 million years ago, the Earth was in two very cold ice ages.



See here for a paper on CosmoClimatology by Henrik Svensmark.
See here for a discussion of the Shaviv and Veizer 2003 paper by Tim Patterson. See here for their paper.


Milankovitch Cycles

The Earth-Sun orbital changes are the principal causes of long term climate change.  During the last 800,000 years, eight periods of glaciations have occurred. Each ice age lasts about 100,000 years with warm interglacial periods lasting 10,000 to 12,000 years. Milutin Milankovitch (1879-1958) identified three major cyclical variables which became recognized as the major causes of climate change. The amount of solar radiation reaching the Earth depends on the distance of the Earth to the Sun and the angle of incidence of the Suns rays upon the Earths surface. The Earths axis tilt changes on a 40,000-year cycle, the precession of the equinox changes on a 21,000-year cycle, and the eccentricity of the Earths elliptical orbit changes on a 100,000-year cycle.

The Earth's axis tilt (also known as obliquity of the ecliptic) changes from 22 to 24.5 degrees over a 40,000-year cycle. Summer to winter extremes are greater when the axis tilt is greater. The precession of the equinox refers to the Earth's wobble as it spins on its axis. Currently, the north axis points to the North Star, Polaris. In 13,000 years it would point to the star Vega, then return to Polaris in another 13,000 years, creating a 26,000-year cycle. When this is combined with the advance of the perihelion (the point at which the Earth is closest in its orbit to the Sun), it produces a 21,000-year cycle. The variation of the elliptical shape of the Earth's orbit around the sun ranges from an almost exact circle (eccentricity = 0.0005) to a slightly elongated shape (eccentricity = 0.0607) on a 100,000-year cycle.  The Earth's eccentricity varies primarily due to interactions with the gravitational fields of other planets. The impact of the variation is a change in the amount of solar energy from closest approach to the Sun (perihelion, around January 3) to the furthest distant to the Sun (aphelion, around July 4).  Currently the Earth's eccentricity is 0.016 and there is about a 6.4 percent increase in incoming solar energy from July to January. In the Northern Hemisphere, winter occurs during the closest approach to the Sun. The graph below shows the three cycles versus time. The vertical line represents the present, negative time is the past and positive time is the future. See here.

orbital variation






Analysis of deep-sea cores shows sea temperature changes corresponding to these cycles, with the 100,000-year cycle being the strongest.

These solar cycles do not cause enough change in solar radiation reaching the Earth to cause the major climatic change without an amplifier effect. A plausible amplifier is the Suns varying solar wind that modifies the amount of cosmic rays reaching the Earths atmosphere.

The rate of change of global ice volume varies inversely with the solar insolation due to orbital changes. The graph below compares the June solar insolation anomaly north of 65 degrees latitude to the rate of change of global ice volume over the last 750,000 years. Reconstructions of global ice volumes rely on the measurement of oxygen isotopes in the shells of foraminifera from deep-sea sediment cores. The records also in part reflect deep ocean temperatures. Two ice records are shown; SPECMAP and HW04.

global ice volume and solar insolation

The ice melting and sublimation rates are very sensitive to summertime temperatures. The strong correlations and the absence of a large time lag demonstrate essentially concurrent variations in the change of ice volumes and summertime insolation in the northern high latitudes. Both ice volume reconstructions therefore support the Milankovitch hypothesis and show that the Sun is the dominant climate driver. The graph is from a paper by G. Roe here.

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